1. Field of Invention
This invention relates to bending glass sheets by the gravity sag technique and particularly, relates to the use of tungsten alloy inserts to support the glass sheets during bending to reduce marking during the glass sheet bending operation.
2a. Technical Considerations
Glass sheets are commonly bent by a gravity sagging technique in which one or more flat glass sheets are mounted on an outline mold comprising a rigid metal rail disposed edge-wise with its upper edge forming a surface that conforms in elevation and outline to the shape desired slightly inboard of the bent glass sheet perimeter. The flat glass sheets are supported at selected points on the mold, usually at the corners. As the glass sheets are heated to their heat deformation temperature, they sag under the force of gravity and conform with the elevational contours of the mold rail. Since the length of the chord between ends of the shaped glass sheet is shorter than the distance between the ends of the flat glass sheet before bending, the glass sheet directly supported by the mold rail must slide over its initial support points. The friction between the hot glass sheet and the hot mold results in scuff marks on the glass surface. In addition, oxidized metal particles from the shaping rail that is in prolonged contact with the hot glass sheet, such as the support points, can become embedded in the glass sheet, resulting in additional visual defects and possible venting of the glass.
After the sheets are shaped, the mold is cooled to minimize excessive sagging of the glass sheets within the outline mold. The shaped glass sheet so cooled is then removed from the mold, inspected, and further processed.
When glass sheets are shaped to relatively deep bends, the length of the chord between the ends of the bent glass sheet are even shorter, which results in additional marking of the glass sheet. In order to reduce this sliding between the glass sheet and the metal rail, the mold is designed to include articulating end rail sections whose contours conform to the outline and shape desired for the longitudinal end portions of the glass sheet to be bent. The end rail section is constructed and counterweighted to pivot downward into an open position to support the mass of the relatively stiff flat glass sheet to be bent and to pivot upward into a closed position when the glass sheet is softened by heat. When in the closed position the end rail sections cooperate with additional shaping rails in the central non-articulating portions of the mold to form a substantially continuous outline shaping surface conforming in contour and elevation to the shape desired for the glass sheet. Although the use of an articulated mold helps reduce scuff marks on the glass sheet surface, there is still sliding of the glass sheet over its initial support points which results in glass sheet marking as well as the previously discussed embedded particle marking.
When a bent glass sheet has excessive marking it is rejected during inspection. It would be useful to find a way to reduce the marking of the glass sheets due to the mold support so as to increase the yield percentage of the bent glass.
2b. Patents of Interest
U.S. Pat. No. 3,184,259 to Almdale relates to steel articles provided with carbide inserts such as screw members used on tongs for handling hot glass sheets in a vertical press bending operation. The invention teaches a method of joining a cemented carbide insert of one or more heavy metals such as tungsten, vanadium, or titanium to an insert supporting steel body.
U.S. Pat. No. 3,976,462 to Sutara teaches the use of thin sheet metal members to cover selected portions of the metal rail in a bending mold that directly contact the glass sheet for an extended period of time during the bending operation. The metal reacts to changes in temperature in the atmosphere in which the glass sheet is bent more nearly like the reaction of the glass sheet itself so as to reduce the high thermal gradient that generally exists between the portion of the glass sheet in contact with critical portions of the mold shaping rail and the other portion of the glass sheet that do not make as long a contact or make no contact whatsoever with the mold rail throughout the heating phase of the bending operation.
U.S. Pat. No. 4,159,842 to Perkowski teaches a self closing tong for vertical press bending wherein the glass engaging member of the tong is composed of a tungsten nickel copper composite essentially free of cobalt and iron. Tongs of this type reduce marring and weakening of the glass sheet in the vicinity where the tongs penetrate the sheet surface.
U.S. Pat. No. 4,119,428 to DeAngelis teaches an adjustable gravity sag bending mold with pairs of cylindrically shaped flat glass supporting members attached longitudinally outward and slightly upward of the transverse end portions of the end shaping rails. The flat glass sheets are positioned on the supporting members and as the glass is heated and begins to sag, the glass sheet surface that is in contact with the supporting members slides over the rounded surface of the supporting members to reduce glass sheet marking at the supports. In addition, the entire mold is tilted a small angle downward to the horizontal to provide resistance to transverse sliding of the glass sheet when the mold is accelerated rapidly at the glass sheet loading station for movement through the bending lehr. The flat glass sheet supporting members are stainless steel rods with a 1/4 inch (0.64 cm.) diameter.